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    <p>What do you mean by 'editor software'? If this is a text file, have you tried existing production-quality editors, before writing your own? If it's a file storing binary data, have you considered using an RDBMS and manipulating its contents using SQL statements?</p> <p>If you absolutely have to write this from scratch, I'm not sure that mmapping is the way to go. Mmapping a huge file will put a lot of pressure on your machine's VM system, and unless there are many editing operations all over the file its efficiency may lag behind a simple read/write scheme. Worse, as you say, you have problems when you want to extend the file.</p> <p>Instead, maintain buffer windows to the file's data, which the user can modify. When the user decides to save the file, traverse sequentially the file and the edited buffers to create the new file image. If you have disk space it's easier to write a new file (especially if a buffer's size has changed), otherwise you need to be clever on how you read-ahead existing data, before you overwrite it with the new contents. </p> <p>Alternatively, you can keep a journal of editing operations. When the user decides to save the file, perform a topological sort on the journal and play it on the existing file to create the new one.</p> <p>For exclusive file access use the file locking of your operating system or implement application-level locking (if only your editor will touch these files). Depending on mmap for exclusive access constrains your implementation choices.</p>
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